1. Field of the Invention
The present invention relates generally to pre-impregnated composite material (prepreg) that is used in making high performance composite parts. More particularly, the invention is directed to providing prepreg that may be cured/molded to form composite parts having high strength, damage tolerance and interlaminar fracture toughness.
2. Description of Related Art
Composite materials are typically composed of a continuous resin matrix and reinforcing fibers as the two primary constituents. The composite materials are often required to perform in demanding environments, such as in the field of aerospace where the physical limits and characteristics of composite parts is of critical importance.
Pre-impregnated composite material (prepreg) is used widely in the manufacture of composite parts. Prepreg is a combination of an uncured resin matrix and fiber reinforcement, which is in a form that is ready for molding and curing into the final composite part. By pre-impregnating the fiber reinforcement with resin, the manufacturer can carefully control the amount and location of resin that is impregnated into the fiber network and ensure that the resin is distributed in the network as desired. It is well known that the relative amount of fibers and resin in a composite part and the distribution of resin within the fiber network have a large affect on the structural properties of the part. Prepreg is a preferred material for use in manufacturing load-bearing structural parts and particularly aerospace composite parts, such as wings, fuselages, bulkheads and control surfaces. It is important that these parts have sufficient strength, damage tolerance and other requirements that are routinely established for such parts.
The fiber reinforcements that are commonly used in aerospace prepreg are multidirectional woven fabrics or unidirectional tape that contains fibers extending parallel to each other. The fibers are typically in the form of bundles of numerous individual fibers or filaments that are referred to as a “tows”. The fibers or tows can also be chopped and randomly oriented in the resin to form a non-woven mat. These various fiber reinforcement configurations are impregnated with a carefully controlled amount of uncured resin. The resulting prepreg is typically placed between protective layers and rolled up for storage or transport to the manufacturing facility.
Prepreg may also be in the form of short segments of chopped unidirectional tape that are randomly oriented to form a non-woven mat of chopped unidirectional tape. This type of pre-preg is referred to as a “quasi-isotropic chopped” prepreg. Quasi-isotropic chopped prepreg is similar to the more traditional non-woven fiber mat prepreg, except that short lengths of chopped unidirectional tape (chips) are randomly oriented in the mat rather than chopped fibers.
The tensile strength of a cured composite material is largely dictated by the individual properties of the reinforcing fiber and matrix resin, as well as the interaction between these two components. In addition, the fiber-resin volume ratio is an important factor. Cured composites that are under tension tend to fail through a mechanism of accumulated damage arising from multiple tensile breakages of the individual fiber filaments located in the reinforcement tows. Once the stress levels in the resin adjacent to the broken filament ends becomes too great, the whole composite can fail. Therefore, fiber strength, the strength of the matrix, and the efficiency of stress dissipation in the vicinity of broken filament ends will contribute to the tensile strength of a cured composite material.
In many applications, it is desirable to maximize the tensile strength property of the cured composite material. However, attempts to maximize tensile strength can often result in negative effects on other desirable properties, such as the compression performance and damage tolerance of the composite structure. In addition, attempts to maximize tensile strength can have unpredictable effects on the tack and out-life of the prepreg. The stickiness or tackiness of the uncured prepreg is commonly referred to as “tack”. The tack of uncured prepreg is an important consideration during lay up and molding operations. Prepreg with little or no tack is difficult to form into laminates that can be molded to form structurally strong composite parts. Conversely, prepreg with too much tack can be difficult to handle and also difficult to place into the mold. It is desirable that the prepreg have the right amount of tack to insure easy handling and good laminate/molding characteristics. In any attempt to increase strength and/or damage tolerance of a given cured composite material, it is important that the tack of the uncured prepreg remain within acceptable limits to insure suitable prepreg handling and molding.
The “out-life” of prepreg is the length of time that the prepreg may be exposed to ambient conditions before undergoing an unacceptable degree of curing. The out-life of prepreg can vary widely depending upon a variety of factors, but is principally controlled by the resin formulation being used. The prepreg out-life must be sufficiently long to allow normal handling, lay up and molding operations to be accomplished without the prepreg undergoing unacceptable levels of curing. In any attempt to increase strength and/or damage tolerance of a given cured composite material, it is important that the out-life of the uncured prepreg remain as long as possible to allow sufficient time to process, handle and lay up the prepreg prior to curing.
The most common method of increasing composite tensile performance is to change the surface of the fiber in order to weaken the strength of the bond between matrix and fiber. This can be achieved by reducing the amount of electro-oxidative surface treatment of the fiber after graphitization. Reducing the matrix fiber bond strength introduces a mechanism for stress dissipation at the exposed filament ends by interfacial de-bonding. This interfacial de-bonding provides an increase in the amount of tensile damage a composite part can withstand before failing in tension.
Alternatively, applying a coating or “size” to the fiber can lower the resin-fiber bond strength. This approach is well known in glass fiber composites, but can also be applied to composites reinforced with carbon fibers. Using these strategies, it is possible to achieve significant increases in tensile strength. However, the improvements are accompanied by a decrease in properties, such as compression after impact (CAI) strength, which requires high bond strength between the resin matrix and fibers.
Another alternative approach is to use a lower modulus matrix. Having a low modulus resin reduces the level of stress that builds up in the immediate vicinity of broken filaments. This is usually achieved by either selecting resins with an intrinsically lower modulus (e.g. cyanate esters), or by incorporating an ingredient such as an elastomer (carboxy-terminated butadiene-acrylonitrile [CTBN], amine-terminated butadiene-acrylonitrile [ATBN] and the like). Combinations of these various approaches are also known.
Selecting lower modulus resins can be effective in increasing composite tensile strength. However, this can result in a tendency to damage tolerance, which is typically measured by a decrease in compressive properties, such as compression after impact (CAI) strength and open hole compression (OHC) strength. Accordingly, it is very difficult to achieve a simultaneous increase in both the tensile strength and damage tolerance
Multiple layers of prepreg are commonly used to form composite parts that have a laminated structure. Delamination of such composite parts is an important failure mode. Delamination occurs when two layers debond from each other. Important design limiting factors include both the energy needed to initiate a delamination and the energy needed to propagate it. The initiation and growth of a delamination is often determined by examining Mode I and Mode II fracture toughness. Fracture toughness is usually measured using composite materials that have a unidirectional fiber orientation. The interlaminar fracture toughness of a composite material is quantified using the G1c (Double Cantilever Beam) and G2c (End Notch Flex) tests. In Mode I, the pre-cracked laminate failure is governed by peel forces and in Mode II the crack is propagated by shear forces. The G2c interlaminar fracture toughness is related to CAI. Prepreg materials that exhibit high damage tolerance also have high CAI and G2c values.
A simple way to increase interlaminar fracture toughness has been to increase the ductility of the matrix resin by introducing thermoplastic sheets as interleaves between layers of prepreg. However, this approach tends to yield stiff, tack-free materials that are difficult to use. Another approach has been to include a tough resin interlayer of about 25 to 30 microns thickness between fiber layers. The prepreg product includes a resin rich surface containing fine, tough thermoplastic particles. For the interlayer-toughened material, even though the initial value of Mode II fracture toughness is about four times as high as that of carbon fiber prepregs without interlayer, the fracture toughness value decreases as the crack propagates and converges at a low value, which is almost the same as that of the non-interleaved system. Ultimately, the average G2c values hit a ceiling as the crack moves from the very tough interlaminar (resin-rich) region of the composite to the less tough intralaminar (fiber) zone.
Although existing prepregs are well suited for their intended use in providing composite parts that are strong and damage tolerant, there still is a continuing need to provide prepreg that may be used to make composite parts that have even higher levels of strength (e.g. tensile strength and compression strength), damage tolerance (CAI) and interlaminar fracture toughness (G1c and G2c).